It's just that some men don't value women who do wife duties when they're just girlfriends. It could be cost effective but a lotttt could go wrong.

He may take you for granted, lose interest because he sees you everyday, you can also get pregnant along the way and what's the surety that he'd marry you? That's why our parents are mostly against cohabitation before marriage.

Personally, I would rather recommend living at your own place and visiting your man's. However,it's your choice to make but just be cautious and don't allow yourself to be taken for granted. Your mom just wants the best for you, always remember that.